1. Field of the Invention
The present invention generally relates to an apparatus for dropping balls into a wellbore. More particularly, the invention relates to an apparatus for dropping one or more balls and that may also be used as an indicator that a plug has been released into a string of drill pipe.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or xe2x80x9chungxe2x80x9d off of the existing casing. Afterwards, the second casing string is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In the process of forming a wellbore, it is sometimes desirable to utilize various plugs. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. Plugs are commonly used, for example, during the cementing operations for a liner.
The process of cementing a liner into a wellbore typically involves the use of both liner wiper plugs and drill-pipe darts. A liner wiper plug is typically located inside the top of a liner, and is lowered into the wellbore with the liner at the bottom of a working string. The liner wiper plug has a cylindrical bore through it to allow passage of fluids. The cylindrical bore also serves as a seat for receiving a drill pipe dart.
After the liner and the attached liner wiper plug is in place, fluid is injected into the wellbore through the working string. The fluid is typically a circulating fluid, or cement. After a sufficient volume of circulating fluid or cement has been placed into the wellbore, the drill pipe dart (sometimes referred to as a pump-down plug) is launched. Using drilling mud, cement, or other displacement fluid, the dart is pumped into the working string. As the dart travels downhole, it seats against the liner wiper plug, closing off the internal bore through the liner wiper plug. Hydraulic pressure above the dart forces the dart and the wiper plug to dislodge from the bottom of the working string and to be pumped down the liner together. This forces the circulating fluid or cement that is ahead of the wiper plug and dart to travel down the liner and to u-turn up into the liner annulus. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner.
The cementing operation described above utilizes a cementing head apparatus at the top of the wellbore for injecting cement and other fluids downhole, and for releasing the plugs. The cementing head typically includes a dart releasing apparatus, referred to sometimes as a plug-dropping container. Darts used during a cementing operation are held at the surface by the plug-dropping container. The plug-dropping container is incorporated into the cementing head above the wellbore. The typical cementing head also includes some mechanism which allows cement or other fluid to be diverted around the dart until plug-release is desired. Fluid is directed to bypass the dart in some manner within the container until it is ready for release, at which time the fluid is directed to flow behind the plug and force it downhole.
The cementing head often includes a plug release indicator, which informs the operator at the surface that a plug has been released. Generally, the release indicator is located below the plug-dropping container and must be reset after each plug is released. In one arrangement, the plug release indicator has a finger that protrudes into the bore of the cementing head. The finger may be xe2x80x9ctrippedxe2x80x9d by a passing plug in the bore to give a positive indication that a plug has been released. The release indicator has an indicator flag located outside of the cementing head that is visible to an operator to indicate release of a plug downhole through the drill pipe.
Plug release indicators are designed to prevent accidental tripping by fluid flow in the bore. Many release indicators use spring washers to resist fluid forces and to maintain the finger in the bore until the released plug trips the finger. However, the setting of the spring washer must be balanced between resisting fluid flow and indicating plug release. If the setting of the spring is too tight, the force required to trip the indicator may be high enough to impede the downward travel of the plug. If the spring setting is too loose, it may be prematurely tripped.
Another common component of a cementing head or other fluid circulation system is a ball dropping assembly for dropping a ball into the pipe string. The ball may be dropped for many purposes. For instance, the ball may be dropped onto a seat located in the wellbore to close off the wellbore. Sealing off the wellbore allows pressure to build up in the wellbore to actuate a downhole tool such as a packer, a liner hanger, a running tool, or a valve. The ball may also be dropped to shear a pin to operate a downhole tool. Balls are also sometimes used in cementing operations to divert the flow of cement during staged cementing operations. Balls are also used to convert float equipment. Thus, multiple balls may be sequentially dropped during a completion operation.
Many ball-dropping assemblies use a retaining device to keep the ball out of the flow stream until release. The retaining device generally includes a plunger that uses linear movement to push the ball into the flow stream at the time of release. These designs tend to extend out from the main body of the cement head, and require numerous manual turns of a wheel to release the ball.
In the assembly of a cementing head, the plug release indicator is typically disposed below the ball dropping assembly in order to verify that a released plug has cleared all possible obstructions in the cementing head. One drawback of this design is that the plug release indicator must be retracted before a ball is released. Additionally, stacking the ball dropping assembly over the plug release indicator increases the length and size of the head member. Furthermore, two different actuators are required to separately actuate a plug release indicator and a ball dropping mechanism.
Therefore, a need exists for a ball dropping assembly that can both drop a ball into the wellbore and indicate that a plug has been released. There is a further need for an apparatus for dropping a ball and for indicating plug release that is more compact, efficient, and inexpensive than using two separate devices for performing these functions. Still further, there is a need for a ball dropping assembly which allows a ball to be dropped into a wellbore without separately retracting a plug release indicator. There is also a need for a combined dart release indicator and ball-dropping apparatus which will reduce the actuator power and control system requirements for remotely controlled operations. Finally, there is a need for such an apparatus that allows for the sequential dropping of more than one ball.
The present invention provides a ball dropping assembly for use in wellbore operations. The novel assembly provides a means for both dropping a ball and for indicating that a plug has been released from a cementing head or other plug-dropping apparatus into a wellbore. The assembly of the present invention first comprises a seat for retaining a ball before it is released. The apparatus further comprises a lever for retaining the ball in the seat. The ball-retaining lever has a first finger and a second finger that together form a L-shaped lever whereby the ball is maintained between the two fingers. The ball dropping assembly also comprises a shaft for turning the lever. The shaft also serves as a pin about which the lever pivots from a ball-retained position to a ball released position.
The assembly is located in a side bore adjacent to the main bore in the cementing head. In the ball retained position, the first finger is disposed in the entrance from the side bore to the main bore, thereby preventing the ball from entering the main bore of the cementing head and dropping into the wellbore. Relative to the first finger, the second finger is disposed within the side bore and over the ball. When the ball is ready for release, the lever is rotated in the direction of the main bore, thereby causing the first finger to protrude into the main bore, and simultaneously causing the second finger to urge the ball to unseat and to enter the main bore. This rotation also moves the first finger into position to indicate plug release. When a plug is released into the bore, it will travel down the main bore and trip the first finger, causing the ball retaining lever to rotate back into the ball retained-position. Rotation of the lever causes the shaft to rotate external to the cement head, providing visual confirmation to the operator of plug release downhole.
In one aspect of the invention, the shaft extends perpendicularly through a housing of the cementing head. Sealingly extending the shaft through both sides of the housing provides a pressure-balanced ball dropping assembly that can be actuated with a small amount of torque. Each end of the shaft has an actuating lever for rotating the shaft. The actuating levers are located outside the cementing head and held in position by a detent in the outer wall of the body of the cementing head. The actuating levers also serve as confirmation means for plug release.
An alternative arrangement for a ball dropping assembly is provided, that permits more than one ball to be selectively dropped into the wellbore. In this arrangement, a ball-feeding channel is provided adjacent to the seat. The first ball to be dropped is loaded onto the seat itself. After the first ball has been dropped, the lever is rotated back towards the ball-feeding channel. A biasing feature is provided in the ball-feeding channel, causing the second ball to be urged onto the seat. The ball dropping procedure may then be repeated. In this way, a plurality of balls may be sequentially dropped during a wellbore completion operation.